Its easiest to say dont get grabbed near the ledge at early percent but I won't because the range is just stupid. At early percent, from like 0-10% marth can chain grab diddy. This means that there is nothing you can do about it anyway. So if you cant stop it till then, your better off holding straight up to better d.i the tipper. While I havent had that situation occur as diddy(it happened many times in melee), the general way to d.i forward throw after that is down and away from marth. It becomes a d.i trap with back throw if I remember correctly, because the d.i for that one is away from marth in the opposite direction.
The good thing is as a diddy main, you can pretty much destroy his lack of oos, and switch between melee and brawl pretty easily. Its pretty good to go ahead and pull out two bananas each time you get a kill, since if he waste time to pick it up, he is wasting his invincibility. and if he picks up the banana, Marth becomes a near non-threat, since he loses the main things he had going for him in disjointed hitboxes, and a freakishly large grab range.
Whenever this marth learns how to play with items better it gets a little harder, but the only thing he was doing was throwing them back at you, so i assumed his item game was bad. The great thing about fighting marth is if you live past 70%, your probably living to very high percent due to having too much damage to be comboed into his tippers.
Aside from that, some of the movement was really creative and I will probably end up stealing that later. The only other thing that may help you in the future is that peanut gun is better to do than offstage up b on marth and roy, (and falco side b) because the timing is really strict, and the punishment for missing is usually a dair for your trouble.
But seriously tell the game and watch to get rid of tapjump, it makes his recovery harder to read, and cost him like 4 stocks in two matches lol.